(1) Field of the Invention
This invention relates to a thermoplastic resin base film for a magnetic recording medium, particularly to a thermoplastic resin base film for a magnetic recording medium which is suitable for high density recording. More particularly, it relates to a thermoplastic resin base film suitable for a metallized film video tape.
(2) Description of the Related Art
A metallized film video tape including a thermoplastic resin film having a magnetic material vacuum-deposited on the surface thereof is capable of super-high-density magnetic recording. Since the magnetic metal layer of the metallized film video tape is very thin, the surface configuration of the base film used is completely reproduced on the surface of the metallized film tape. Accordingly, if the surface roughness of the base film is large, the electromagnetic transformation performances of the metallized film video tape are extremely reduced.
In view of the electromagnetic transformation performances, a base film having a smooth mirror surface is preferred. However, since a vacuum-deposited metal film formed on such a base film has the same mirror surface as the base film, the slip characteristics of the metal layer surface are extremely poor. Unless, for example, a protective film layer is formed on the vacuum-deposited metal film, many scratches are inevitably produced on the surface of the metal film in the tape-forming process after the vacuum evaporation step, rendering the metallized film video tape useless for practical purposes.
The running property of the vacuum-deposited layer side of the metallized film video tape is another serious problem during the practical use thereof. In the case of a video tape of the coating type, which is prepared by coating a base film with an organic polymer binder having a magnetic powder incorporated therein, it is possible to improve the running property of the magnetic layer side of the tape by incorporating a lubricant into the organic polymer binder. On the other hand, in the case of a video tape of the metal vacuum deposition type, the running property of the vacuum-deposited layer side is generally inferior unless a protective film layer is formed on the metal film layer. It is, however, difficult to form a uniform protective film layer on the surface of the vacuum-deposited metal film. Therefore, for a metallized film video tape in which no protective film layer is formed, it is impossible to improve the running property of the vacuum-deposited layer surface, especially under the severe conditions during practical use thereof.
As a base film in which the foregoing defect is eliminated, there is the base film described in Japanese Unexamined Patent Publication (Kokai) No. 58-62826. This base film includes a polyester film having on at least one surface thereof a discontinuous film composed mainly of a water-soluble polymer and having a thickness of not larger than 500 .ANG.. The discontinuous film imparts excellent smoothness, slip characteristics, and scratch resistance to a magnetic metal thin film formed thereon. This base film is, however, disadvantageous in that the slip characteristics and scratch resistance are degraded under high temperature and high humidity conditions.
This defect can be eliminated by incorporating fine particles into the discontinuous film. However, it was found that when a metallized film video tape including such a base film, is subjected to repeated recording and reproduction operations, for example, 1000 times or more, the fine particles are liable to fall off from the discontinuous film and peeling is liable to occur of the vacuum-deposited magnetic metal thin film layer from the discontinuous film layer and of the discontinuous film from the base film surface might occur.